At the University of Washington Primate Research Center, several macaque species show an acquired immunodeficiency syndrome (simian AIDS, SAIDS) characterized by lymphocytopenia, opportunistic infections, and a retroperitoneal fibromatosis (RF) tumor. Numerous type-D retroviruses, designated SAIDS-D/Washington (SAIDS-D/W), have been isolated by cocultivation of tissues and blood from animals with RF on lymphocyte and monolayer cultures. This virus has been molecularly cloned; the restriction enzyme pattern reveals that it can be distinguished from all other type-D retroviruses. Epidemiological studies reveal that over 90% of colony animals have antibodies that cross-react with SAIDS-D/W viral proteins. A survey of macaques bled in Indonesia reveals that many of these animals are already antibody positive in their natural habitat. Another retrovirus has been isolated on lymphocyte cell lines after cocultivation of a lymph node from a Macaca nemestrina that had died with lymphoma in 1982 at the Washington Primate Center. This isolate, designated SIV/Mne (simian immunodeficiency virus, M. nemestrina), is partially related to human immunodeficiency virus (HIV, formerly HTLV-III/LAV) as evidenced by an immunological cross-reaction of the major gag protein. SIV/Mne is even more closely related to the west African AIDS isolate, HIV-2, with a 90% amino acid homology in the gag region of the virus. Nine independent molecular clones of SIV/Mne have been obtained and are being characterized. SAIDS- D/W and SIV/Mne have been inoculated into several primate species; the former virus causes RF tumors and the latter causes severe immunosuppression with absolute depletion of T4+ lymphocytes.